Accelerometer-based systems and methods and quantifying steps

ABSTRACT

Systems and methods detect steps from one or more sensed accelerometer signals. Systems comprise an accelerometer and a non-transitory computer readable medium, each of which communicates with a processor. The accelerometer is coupled to an individual and generates outputs received by the processor. The non-transitory computer readable medium stores instructions controlling the processor to perform steps of a method. The processor determines a minimum and a maximum reading for respective time periods. Counted peak heights are maxima that exceed a rest threshold. The rest threshold may be a function of the rest maximum. The processor determines a peak threshold for each time period having a counted peak height. A counted amplitude comprises the difference between the maximum reading and minimum readings of a time period in which the counted peak height exceeds the peak threshold. The processor increments a step counter when a counted amplitude exceeds an amplitude threshold.

This application claims the benefit of U.S. Provisional Application No.62/074,152 filed Nov. 3, 2014 and of U.S. Provisional Application No.62/106,450 filed Jan. 22, 2015, the entire contents of which are herebyincorporated by reference.

FIELD OF THE INVENTION

Systems and methods relate to detecting steps using an accelerometer.

BACKGROUND

Physical activity has long been recognized as benefiting health.Physical activity levels can also be an indicator of health. For thesereasons, a variety of individuals and institutions are interested inmonitoring physical activity. Over the past years, activity trackershave become an increasingly popular way for individuals to monitor theirphysical activity. An activity tracker typically includes anaccelerometer as a means of detecting the physical activity of theindividual wearing it. The accelerometer generates a series of outputsthat vary according to the magnitude of acceleration the accelerometerdetects. Outputs may then be converted into readings that correspond toa particular level of acceleration. A series of readings over time maybe viewed as a signal.

Activity trackers use accelerometer signals to quantify physicalactivity in a variety of units including calories burned, distancetraveled, and steps taken. Devices that count steps are referred to,generally, as pedometers, though accelerometer-based step counters aresometimes distinguished from pedometers that count the oscillations of aweight. The initiation of contact between a foot and the ground during astep causes a peak in the acceleration signal from an accelerometercoupled to the individual taking the step. However, not all peaks in theacceleration signal are caused by steps. Peaks that are not caused bysteps may be referred to as “noise.” Noise may be caused, for instance,by the vibrations of a car or the lack of damping in the accelerometeritself.

One method of filtering out noise is to set an acceleration threshold.Peaks that exceed the threshold are presumed to be caused by steps whilereadings below the threshold are presumed to be noise. However, theboundary between acceleration caused by step activity and that caused bynoise may change substantially depending on factors such as how fast anindividual is walking or running, the elasticity of the surface on whichthe step is taken, and the hardness of the individual's shoe soles.

The graph in FIG. 1 illustrates the shortcomings of a single, fixedthreshold. Two samples of accelerometer gait data are superimposed ontop of each other. Each sample illustrates two steps taken by anindividual. The sample with higher peaks (110) comes from a healthyindividual. The sample with the lower peaks (120) comes from a patientafter surgery. The time scale on the x axis is in 24ths of a second andthe units of acceleration on the y axis are 1/64 of g (g=free-fallacceleration in Earth's gravity). Threshold 1 (130) would record bothsteps from the healthy individual but would not capture any steps fromthe post-surgery patient. Threshold 2 (140) records both steps from thepost-surgery patient, but counts four steps from the healthy individualwhen only two steps were actually taken. Threshold 3 (150), justslightly lower than Threshold 2, also records both steps from thepost-surgery patient, but counts only one step from the healthyindividual. Thus, a fixed threshold does not accurately count steps fromboth the patient and healthy individual.

SUMMARY

Embodiments include systems and methods for detecting steps from one ormore sensed accelerometer signals. Systems comprise an accelerometer anda non-transitory computer readable medium, each of which communicateswith a processor. The accelerometer generates outputs received by theprocessor and is coupled to an individual whose steps are to bedetected. The non-transitory computer readable medium storesinstructions controlling the processor to perform steps of a method. Theprocessor determines readings from the accelerometer outputs. Thedetermination may be, for example, multiplying each output by acoefficient or processing the accelerometer outputs with a datasmoothing function. In some embodiments, a peak height is the greatestreading from a time period. Counted peak heights are those peak heightsthat exceed a rest threshold. The rest threshold may be determined byreceiving readings from the accelerometer when it is at rest for aperiod of time, identifying the rest maximum (the greatest readinggenerated during the period of rest), and then determining the restthreshold using the rest maximum. The processor determines a peakthreshold for each time period having a counted peak height. The peakthreshold comprises a weighted average of two or more counted peakheights. A counted amplitude comprises the difference between themaximum reading and minimum reading during a time period in which thecounted peak height exceeds the peak threshold. The processor incrementsa counter when a counted amplitude exceeds the amplitude threshold forthe time period of the counted amplitude. The amplitude threshold for agiven time period comprises a weighted average of two or more countedamplitudes. An increment represents a step taken by the individual andthe number stored by the counter represents a number of steps taken bythe individual.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the shortcomings of fixed thresholds for countingsteps.

FIGS. 2a-2c are schematic diagrams illustrating systems according tocertain embodiments of the present disclosure.

FIG. 3 illustrates a schematic diagram of a system for counting stepsaccording to certain embodiments.

FIGS. 4a-4c illustrate three approaches to defining peak height andamplitude.

FIGS. 5a and 5b illustrate accelerometer readings at rest.

FIG. 6 is a flowchart illustrating a computer-implemented method forcounting steps using peak height thresholds according to certainembodiments of the present disclosure.

FIG. 7 is a flowchart illustrating a computer-implemented method forcounting steps using amplitude thresholds according to certainembodiments of the present disclosure.

FIG. 8 is a flowchart illustrating a computer-implemented method forcounting steps using multiple thresholds according to certainembodiments of the present disclosure.

FIG. 9 illustrates a rest threshold applied to segments of anaccelerometer signal according to certain embodiments of the presentdisclosure.

FIG. 10 illustrates a peak threshold applied to segments of anaccelerometer signal that have exceeded a rest threshold according tocertain embodiments of the present disclosure.

FIG. 11 illustrates an amplitude threshold applied to segments of anaccelerometer signal that have exceeded both a peak threshold and a restthreshold according to certain embodiments of the present disclosure.

DETAILED DESCRIPTION

Systems and methods detect steps by applying sequential thresholds toone or more sensed accelerometer signals. Sequential thresholds considersignal values that exceeded a first threshold for comparison to a secondthreshold. In some embodiments, the second threshold may be a functionof one or more values that exceeded the first threshold. By sensingchanging signal values of the accelerometer readings, the systems andmethods dynamically change the thresholds over time.

Systems:

The step-counting methods described herein are performed by a systemthat includes an accelerometer generating outputs. An accelerometer mayrefer to any device that measures either linear or angular acceleration,though accelerometers measuring angular acceleration may be referred toas gyroscopes or simply gyros. An accelerometer may refer to a devicethat measures acceleration in more than one direction. Triaxialaccelerometers, for example, measure acceleration on three axes that maybe at least approximately orthogonal to one another. In addition tolinear and angular acceleration, some accelerometers also contain amagnetometer that can orient accelerations relative to a magnetic fieldsuch as Earth's magnetic field. A so-called “9-axis accelerometer”contains a tri-axial accelerometer measuring linear acceleration alongthree orthogonal axes, angular acceleration around each of theorthogonal axes, and orientation relative to a magnetic field. A 9-axisaccelerometer may also be referred to as an inertial measurement unit(IMU).

The accelerometer may be coupled to an individual whose steps are to bemeasured. Preferred embodiments measure linear acceleration along atleast the anterior-posterior axis of the individual, which isapproximately vertical when the individual is walking upright. Use oftriaxial accelerometers may be advantageous because the accelerometerneed not be oriented precisely when coupled to the individual. Summingreadings from each of the three axes may produce a signal from whichsteps may be detected without orienting the accelerometer relative tothe individual or to the direction of gravity. Alternatively, for atriaxial accelerometer, the direction of gravity may be deduced from thecomponent of constant acceleration detected at each of the three axesand then virtually oriented relative to gravitational pull.

The accelerometer may be communicatively coupled to a processor. Theprocessor may include one or more processing units. The processor iscommunicatively coupled to and executes instructions from a computerreadable data storage (CRDS) (also referred to herein as anon-transitory computer-readable medium). Embodiments of the CRDS mayinclude random access memory (RAM) and various types of non-volatilememory including, but not limited to, read-only memory (ROM),programmable read-only memory (PROM), erasable programmable read-onlymemory (EPROM), and electrically erasable programmable read-only memory(EEPROM). Embodiments of the CRDS may include magnetic storage (such ashard drives, floppy disks, and magnetic tape) and/or optical storage(such as CDs and DVDs).

If the method uses time periods, the system may include a means formeasuring time. Because processors typically perform calculations on aregular time cycle, some embodiments count processor cycles as a meansfor measuring time. If the accelerometer generates outputs at regulartime intervals, embodiments may count the number of accelerometeroutputs to measure the passage of time. Alternatively, a separatetime-measuring device may be used to measure time periods. Examples ofalternative time measuring devices include electric clocks such asquartz clocks, synchronous clocks, and radio-controlled clocks that arewirelessly synchronized with a time standard such as an atomic clock.

The output of the system is a signal indicating that a step has beentaken. The system may store, on the CRDS for example, a tally of thenumber of steps that have been taken during a specified amount of time.The processor may increment the tally each time a step is detected sothat the tally serves as a step counter. The system may also include ameans for communicating these results. The communication means may be,for example, a visual display, audio speaker, or tactile output. Thesystem may also include a user interface allowing a user to inputinformation into the computer, for example to display different measuresof physical activity.

Certain components of the system may be communicatively coupled to othercomponents. The processor, for instance, may receive information fromthe accelerometer, send information to the communication means (if any),and both send and receive information from the CRDS. Thesecommunications may be made without physical connections such as wires.Any of the communications may be accomplished wirelessly using signaltypes including radio and infrared. Common wireless communicationprotocols include Bluetooth (IEEE 802.15), Wi-Fi (IEEE 802.11), cellularcommunication protocols, and infrared data association protocols.Embodiments using wireless communications may include components such astransmitters and receivers. Systems may also communicate with otherdevices or networks, for example a personal computer or the Internet.

FIGS. 2a through 2 c illustrate accelerometer systems for counting stepsaccording to the present disclosure. FIG. 2a illustrates a schematicdiagram of the system (200) that includes an accelerometer (210) thattransmits outputs to the processor (220). The dashed line connecting thetwo components indicates that they are communicatively coupled;information may be transferred from one to another, but does notnecessarily require a physical connection. The processor (220) executesinstructions stored on a computer-readable data storage (CRDS) (230).The processor (220) may store and retrieve data, such as accelerometerdata, from the CRDS (230) and processes the data to detect steps. Theprocessor (220) may communicate step data or values derived from thestep data to a communication means (240). The components listed aboveare powered by a power supply (250). While this diagram shows one powersupply providing energy to all of the components, the system (200) mayuse multiple power sources. Any configuration of power sources may beused provided that each component receives power, either directly from apower supply or indirectly through other components.

FIG. 2b illustrates a schematic diagram of a system (201) that may beimplemented as a portable, self-contained activity tracker according tocertain embodiments. The rectangle surrounding the components (271)indicates that they are all housed in the same device. The activitytracker (271) may include a 3-axis accelerometer (211), a processorconfigured to function in an activity tracker (221), random accessmemory (RAM) (231), a display screen (241) visible from the outside ofthe device, and a rechargeable lithium-ion (Li-Ion) battery (251). Theprocessor (221) performs calculations on the acceleration data andstores any necessary information on the RAM (231). The display (241) maygraphically display, among other information, step counts over varioustime periods. From time to time, the activity tracker (271) maycommunicate with a second device (281) via radio signals. The activitytracker (271) contains a Wi-Fi transmitter-receiver (261) that exchangesdata with the external device (281). This wireless communication allowsthe activity tracker (271) to transmit unprocessed accelerometer data orstep counts to the external device (281) which may be communicativelycoupled to networks such as the Internet, and to receive updatedinstructions.

FIG. 2c illustrates a schematic diagram of a system (202) in which amotion monitor (272) wirelessly communicates accelerometer outputs to anexternal device (282) for processing, according to certain embodiments.In this embodiment, the external device is a smartphone (282). Themotion monitor (272) contains a 9-axis accelerometer/IMU (212), aBluetooth transmitter (262), and an ambient energy collector (252) topower the other two components. The ambient energy collector (252) may,for example, capture energy from the user's body heat. The Bluetoothtransmitter (262) transmits unprocessed accelerometer outputs to aBluetooth receiver (263) in the smartphone (282). The Bluetooth receiver(263) then relays the accelerometer outputs to the processor (222)which, in turn, stores and retrieves acceleration data from electricallyerasable programmable read-only memory (EEPROM) (232) and determines astep count. The processor (222) then communicates the step count via thedisplay screen (242) on the smartphone (282). The components in thesmartphone (282) may be powered by a nickel metal-hydride (NiMH) battery(253).

FIG. 3 illustrates a schematic diagram of a system (300) for countingsteps, according to certain embodiments. An accelerometer (210)transmits acceleration data to the processor (320). The processor (320)stores and retrieves data from a CRDS (230) and may send signals to adisplay screen (242). The processor (320) comprises circuitry operableto perform steps for determining a step count including determination ofpeak height for a time period (322), comparison of the peak height to apreviously determined rest threshold (323), determination of a peakthreshold (324), comparison of the peak height to the peak threshold(325), determination of an amplitude for a time period (326),determination of an amplitude threshold (327), comparison of theamplitude for the time period with the amplitude threshold (328), anddetermination of a step count (329). Two of the steps, determination ofpeak height (322) and determination of amplitude (326), may use outputsfrom the accelerometer (210). Outputs from the accelerometer (210)during a time period may be compared to determine peak height (322),according to a definition of peak height including those discussedherein. Readings from the accelerometer (210) may be compared todetermine trough height, according to a definition of peak heightincluding those discussed herein. The processor (320) may then subtracttrough height from the peak height from the same time period todetermine amplitude (326). Readings from the accelerometer may also beused to determine values such as a rest maximum.

Each of the processor tasks of FIG. 3 may involve communication betweenthe processor (320) and the CRDS (230). Readings for a given time periodmay be stored on the CRDS (230) and accessed by the processor (320) todetermine peak height (322). Peak heights and the rest threshold may bestored in the CRDS (230) to allow comparison of the two values (323).Counted peak heights stored on the CRDS (230) may be used by theprocessor (320) to calculate a peak threshold (324). Comparison of thepeak height to the peak threshold (325) may involve storage of the twovalues being compared on the CRDS (230). Readings from a time periodstored on the CRDS (230) may be compared to determine a trough heightwhich, when subtracted from the peak height for the time period, yieldsthe amplitude for the time period (326). Counted amplitudes stored onthe CRDS (230) may be accessed by the processor (320) for calculation ofthe amplitude threshold (327). The processor (320) may compare anamplitude for a time period and the amplitude threshold for the timeperiod stored on the CRDS (230) to determine if the amplitude exceedsthe threshold (328). The CRDS (230) may also be used to store a totalnumber of steps. When the processor (320) determines that a step hasbeen taken (329), it may increment the total step count stored on theCRDS (230). The time periods determined by the system (300) may bemeasured by an electric clock (350).

Acceleration Readings from Accelerometer Outputs:

The output of an accelerometer may be analog or digital. Digitalaccelerometers may produce a binary output of either a high or lowvoltage and emit outputs at a particular frequency. In some embodiments,the accelerometer output is expressed as a binary number. The output ofan analog accelerometer is typically an electrical signal whose voltageis related to the acceleration detected by the accelerometer. As aresult, accelerometer outputs may be processed to yield accelerationreadings (or simply “readings”). If the output voltage of anaccelerometer is directly proportional to the acceleration itexperiences, an accelerometer output may simply be multiplied by acoefficient to yield an acceleration reading. If the output voltage isdirectly proportional to acceleration, the accelerometer output may betreated as the reading itself if the particular units of accelerationare not critical to the purpose of the accelerometer. Thus, in someembodiments, accelerometer outputs function as readings without furtherprocessing. If the relationship between accelerometer output andacceleration is non-linear, a function more closely matching therelationship may be used to convert the output into acceleration.

Data Smoothing:

Some embodiments use a type of process referred to as data smoothing.Data smoothing includes a variety of techniques to remove noise fromsignals and may be applied either before or after accelerometer outputsare converted into readings. However, as used herein, the output of adata smoothing function may be referred to as a reading even if thereadings are the input to a data smoothing function. Some embodimentsuse a low pass filter to remove signal components that are not likelycaused by steps. In some embodiments, the low pass filter converts theaccelerometer signal from the time domain to the frequency domain usinga Fourier transform. Component frequencies too high to be caused by stepmotion may be removed from the signal in the frequency domain beforeconverting the filtered signal back to the time domain. Some embodimentssmooth the accelerometer signal using a moving average in which thereading is a function of an average or weighted average of the n mostrecent readings. As n increases, the signal becomes increasingly smooth.In some embodiments, n is high enough to remove noise in the signal butlow enough that features of interest, such as steps, are not lost. Someembodiments use curve fitting to remove noise from the accelerometerdata. Curve fitting creates an equation that approximates theaccelerometer signal and the equation outputs are treated as readings.

Definitions of Peak Height and Amplitude:

Peak height and amplitude are two signal features to which thresholdsmay be applied. As used herein, peak height is the highest reading ofthe accelerometer within a time period and amplitude is the differencebetween the highest reading and the lowest reading within a time period.However, different definitions of peak height and amplitude specifydifferent time periods.

Some embodiments have time periods that are of a predetermined length.The length may be the same for all time periods, or time periods may beof different (but still predetermined) lengths. Alternatively, someembodiments have variable time periods that are determined by the datasignal. For example, some embodiments set time periods that begin andend each time accelerometer readings increase above a threshold. Someembodiments define time periods as the time between a local maximum andan adjacent local minimum. Time periods may be discrete or continuous.Discrete time periods may be contiguous, separated, or overlapping.Continuous time periods may be described as “sliding windows”-alwaysconsidering a particular number of readings or readings from a fixedamount of time before a particular reading.

FIGS. 4a-4c illustrate three approaches to defining peak height andamplitude. The time scale on each x axis is in 1/24ths of a second andthe units of acceleration on the y axis are 1/64^(th) of g (g=free-fallacceleration in Earth's gravity). FIG. 4a defines peak height andamplitude relative to uniform, contiguous time periods. In thisembodiment, the time period is constant and set at ⅓ (8/24) of a second.The boundaries between time periods are represented by the verticaldashed lines. The peak height and the trough height are the highest andlowest points, respectively, within each time period and are representedby horizontal dashed lines. The length of the vertical dotted linesbetween the peak height and the trough height represents the amplitudefor each time period. FIG. 4b illustrates peak height and trough heightas defined by a threshold. In this embodiment, the threshold has beenset at 80/64 of g. The peak height is the highest point between twothreshold crossings that is above the threshold and the trough height isthe lowest point between two threshold crossings that is below thethreshold. Peak heights and trough height are represented by horizontaldashed lines and the amplitudes are represented by the lengths of thevertical dotted lines. FIG. 4c illustrates the use of local maxima andminima as peak heights and trough heights, respectively. A readingflanked by lower readings on both sides is a local maximum. Conversely,a reading flanked on both sides by higher readings is a local minimum.In this embodiment, an amplitude is the difference in height between apeak height and a directly adjacent trough height. Again, peak heightsand trough heights are represented by horizontal dashed lines andamplitudes are represented by the lengths of the vertical dotted lines.

Time Period:

For embodiments that determine peaks and/or troughs according topredetermined time periods, maximum step count is one factor to considerwhen selecting the length of the time periods. Each time period eitherincreases the step count by one or does not. As time periods becomeshorter, the potential count over a standard unit of time (such as aminute or hour) increases. For example, dividing time periods into ⅓ ofa second allows for a maximum of 180 steps recorded in a minute.Research indicates that 180 steps per minute is approximately themaximum a human could achieve, though most people do not approach thislimit. Dividing seconds into halves would allow for a maximum of only120 steps per minute. Preferred embodiments use time periods of aquarter to three quarters of a second, though embodiments with timeperiods outside this range may also be useful.

Baseline Readings:

Baseline readings are those readings an accelerometer would be expectedto generate when it is at rest. Gravity exerts a force on stationaryobjects as if they were accelerating at a constant rate. Anaccelerometer at rest may yield a baseline reading close to g (Earth'sgravitational acceleration) if the accelerometer is oriented parallel tothe force of gravity. However, a resting accelerometer orientedperpendicular to gravitational force would be expected to yield abaseline reading close to zero. Thus, the baseline reading of anaccelerometer at rest is expected to be between zero and g depending onthe accelerometer's orientation relative to the force of gravity.

Rest Threshold:

Many accelerometers generate fluctuating readings when they are expectedto generate a constant baseline reading. These fluctuations may be dueto noise inherent in the accelerometer. Rest thresholds are one way ofscreening out this inherent noise. A rest maximum is the highest readingthat an accelerometer generates when it is at rest for a period of time.The rest maximum may be due to both noise inherent in the accelerometerand the force of gravity.

FIGS. 5a and 5b illustrate two views of a data sample collected from anaccelerometer at rest for 10 seconds. The time scale on the x axis is in24ths of a second and the units of acceleration on the y axis are64^(ths) of g. The primary difference between the two graphs is thescale on the y axis. FIG. 5a has a range from 0 to 100/64 of g whileFIG. 5b is magnified to show only the range between 55/64 and 85/64 ofg. The average of the readings during these 10 seconds is approximatelyone (64/64) g. This is due to the force of Earth's gravity on theaccelerometer. However, readings fluctuate from as low as 58/64 g to ashigh as 72/64 g. Thus 72/64 g would be the rest maximum for thisparticular ten-second sample.

Some embodiments use the unaltered rest maximum as the rest threshold.However, some embodiments set the rest threshold above the rest maximumbecause the resting accelerometer may only be observed for a finiteduration. If the accelerometer were observed for a longer duration, ahigher rest maximum would likely be observed. The increase could beachieved in a variety of ways including multiplying the rest maximum bya coefficient greater than or equal to one, and/or by adding a specifiedamount to it. The rest threshold may be determined once and need not bedetermined repeatedly. The rest threshold may, for example, bedetermined at the factory at which the system is manufactured and storedin the CRDS of the system.

Peak Thresholds:

Some embodiments further filter noise by comparing peak heights with apeak threshold. The peak threshold may comprise a function of one ormore previous peak heights. In some embodiments, the function is ameasure of central tendency applied to two or more previous peakheights. For example, the function may take the mean, median, or aweighted average of two or more previous peaks.

In some embodiments, previous peak heights are the most recent peakheights. Consideration of the most recent peak heights allows the peakthreshold to be adjusted for the most recent conditions. Taking aweighted average that weights more recent peaks more heavily furtheremphasizes the effects of recent changes on the threshold. The peaksneed not be the very most recent; taking the average of the first fiveof the most recent six peaks, for example, would account for very recentpeaks without using the most recent one.

Some embodiments use thresholds in combination with other thresholds.When determining a peak threshold, some embodiments calculate a functionof only those peak heights exceeding a first threshold. A peak thatsatisfies one or more criteria for further consideration is referred toa counted peak. When calculating peak thresholds, some embodiments takea function of only two or more counted peaks. In some embodiments,counted peaks comprise only those that exceed the rest threshold. Insome embodiments, the threshold is a function of amplitude, discussedherein.

As with the rest threshold, there may be a need to adjust a peak heightor threshold to which it is compared. Adjusting the threshold or thepeak height can make a test more or less sensitive. Adjusting the peakor peak threshold may be achieved, for example, by multiplying peakheight (of either the focal peak or the peaks influencing the thresholdto which the focal peak is compared) by a coefficient or adding orsubtracting an amount from it. A peak height that has been adjusted maybe referred to as an adjusted peak height. When determining peakthresholds, some embodiments take a function of only two or moreadjusted peak heights.

FIG. 6 illustrates a flowchart of a method (600) of counting steps thatapplies peak thresholds to accelerometer data, according to certainembodiments. The accelerometer is part of a system, such as systems200-202 and 300, and is communicatively coupled to a processor capableof executing the instructions to process the accelerometer data. Theprocessor first determines whether the maximum reading from a timeperiod is greater than a rest threshold (610). The rest threshold (620)is a function of the rest maximum. The function may, for example,multiply the rest maximum by a coefficient slightly greater than one.The rest threshold (620) may be determined once and need not bedetermined each time the method (600) is executed. If the maximumreading for the time period is not greater than the rest threshold,processing proceeds to the next time period (630) unless the method(600) is complete (640). If the maximum reading for the time period isgreater than the rest threshold, the maximum is referred to as a countedpeak height and the processor determines an adjusted peak height as afunction of the counted peak height (650). The adjustment function maybe, for example, to multiply the peak height by a coefficient or add afixed value. The method then determines whether the maximum peak heightfor a given time period is greater than the average of the most recentfive adjusted peak heights (660). If not, processing proceeds to thenext time period (630) unless the method (600) is complete (640). If themaximum from the time period is greater than the average of the lastfive adjusted peak heights, a repetition counter is incrementedindicating that a step has been taken (690). Then, processing proceedsto the next time period (630) unless the method (600) is complete (640).

Amplitude Thresholds:

Thresholds for amplitude offer another means of filtering noise. Theanalysis of amplitude parallels the analysis of peak height in manyways. An amplitude from a time period is may be compared to a functionof one or more past amplitudes. Embodiments may use, for example, a meanor weighted average of past amplitudes or other measure of centraltendency. The amplitudes may be adjusted to make the test more or lesssensitive. Adjustments may be made either to the focal amplitude or toone or more past amplitudes influencing the threshold to which the focalamplitude is compared. Adjustments would include multiplying amplitudesby a coefficient or adding or subtracting an amount from an amplitude.

Some embodiments use amplitude thresholds in combination with one ormore other thresholds. Some embodiments consider only those amplitudeswhose peak heights exceed the rest threshold. Some embodiments use otherthresholds to screen the amplitudes that affect the amplitude threshold.For example, some embodiments consider only those amplitudes from timeperiods for which the peak height exceeds a peak threshold. An amplitudethat meets one or more criteria for further consideration may bereferred to a counted amplitude. For example, counted amplitudes may bethose amplitudes from time periods whose peak heights exceeded a peakthreshold. For some embodiments, amplitude thresholds may be a functiononly of counted amplitudes.

FIG. 7 illustrates a flowchart for a method (700) of counting steps thatcomprises an amplitude threshold in combination with a rest threshold,according to certain embodiments. An accelerometer is part of a system,such as systems 200-202 and 300, and is communicatively coupled to aprocessor capable of executing the instructions to analyze theaccelerometer data. In this embodiment, accelerometer outputs areprocessed with a data smoothing function to yield readings (705). Theprocessor then determines whether the maximum reading from a time periodis greater than a rest threshold (610). The rest threshold (620) is afunction of the rest maximum which is the highest reading generated bythe accelerometer while it is at rest for a period of time. The restthreshold (620) may be determined once and need not be determined eachtime the method (700) is executed. If the maximum reading for the timeperiod is not greater than the rest threshold, processing proceeds tothe next time period (630) unless the method is complete (640). If themaximum reading for the period is greater than the rest threshold, thenthe method calculates the difference between the maximum and minimumreadings for the time period, the difference referred to as a countedamplitude (750). The adjusted amplitude is a function of the countedamplitude (760). The program then determines whether the amplitude fromthe time period is greater than the weighted average of the last fouradjusted amplitudes (770). If not, processing proceeds to the next timeperiod (630) unless the method is complete (640). If the amplitude isgreater than the average of the last four adjusted amplitudes, arepetition counter is incremented indicating that a step has been taken(690). Then, processing proceeds to the next time period (630) unlessthe method is complete (640).

Combinations of Thresholds:

Thresholds may be used in combination to filter noise from theaccelerometer signal. There are multiple ways in which thresholds may becombined. Embodiments with simultaneous combinations simply identifysignal features, such as a peak heights or amplitudes, that are greaterthan the higher of two or more thresholds. In sequential embodiments,one threshold is used as a criterion for inclusion of a value in afunction to create a second threshold. A feature that meets one or morecriteria is referred to a counted feature. A threshold may be calculatedusing counted features rather than all features of a particular type.Additionally, a counted feature may also serve as the metric ofinterest, such as a step.

FIG. 8 illustrates a flowchart of a method (800) for sequentiallycombining rest, peak height, and amplitude thresholds to count steps,according to certain embodiments. An accelerometer is part of a system,such as systems 200-202 and 300, and is communicatively coupled to aprocessor capable of executing the instructions to analyze theaccelerometer data. In this embodiment, accelerometer outputs areprocessed with a data smoothing function to yield readings (705). Datasmoothing functions could include, for example, a low pass filter, amoving average, or a curve fitting function. The processor thendetermines whether the maximum reading from a time period is greaterthan a rest threshold (610). The rest threshold (620) is a function ofthe rest maximum which is the highest reading generated by theaccelerometer while it is at rest for a period of time. The functionmay, for example, multiply the rest maximum by a coefficient slightlygreater than one. The rest threshold (620) may be determined once andneed not be determined each time the method (800) is executed. If themaximum reading for the time period is not greater than the restthreshold, processing proceeds to the next time period (630) unless themethod (800) is complete (640). If the maximum reading for the timeperiod is greater than the rest threshold, then the program identifiesthe maximum as a counted peak height and then executes a function of thecounted peak height, the output of the function referred to an adjustedpeak height (650). The processor then determines whether the maximumpeak height for the time period is greater than the average of the lastfive adjusted peak heights (660). If not, processing proceeds to thenext time period (630) unless the method (800) is complete (640). If themaximum from the time period is greater than the average of the lastfive adjusted peak heights, then the processor calculates a countedamplitude as the difference between the maximum and minimum readings forthe time period (750). In some embodiments, the amplitude may beadjusted by a function. The processor then determines whether theamplitude from the time period is greater than the average of the lastfive counted amplitudes (870). If not, processing proceeds to the nexttime period (630) unless the method (800) is complete (640). If theamplitude is greater than the average of the last five countedamplitudes, a repetition counter is incremented indicating that a stephas been taken (690). Then, processing proceeds to the next time period(630) unless the method (800) is complete (640).

FIGS. 9, 10, and 11 include data plots illustrating how the method ofFIG. 8 would be applied to a sample of accelerometer data collected over8 seconds. The x-axis in each plot represents time in seconds and timeis partitioned into contiguous periods of one third (⅓) of a second asrepresented by the black vertical lines. The y-axis representsacceleration in g (free fall acceleration in Earth's gravity). The wavysignal line (910) represents the accelerometer reading at each point intime. In this embodiment, peak height is defined as the highest readingin each time period and the amplitude is the difference between thehighest and lowest points in each time period.

FIG. 9 illustrates the application of a rest threshold to a segment ofaccelerometer data, according to certain embodiments. In thisembodiment, the rest threshold has been set at 1.12 g and is representedby the horizontal line (920) at that level. Any time period in which thesignal line does not exceed the rest threshold is eliminated fromconsideration for either a step or calculation of subsequent thresholds.In this particular data segment, the first full time period (930) wasthe only time period in which the signal did not exceed the restthreshold. Time period 930 is shaded to indicate that it has beeneliminated from consideration, both for being counted as a step and inthe calculation of subsequent thresholds. The remainder of the peaks arereferred to as counted peaks.

FIG. 10 illustrates the application of a peak threshold to the datasegment (910) after the application of a rest threshold, according tocertain embodiments. The horizontal dashed line in each time period(e.g. 1020) represents the peak threshold for that period. Each peakthreshold is the mean of the last five counted peak heights each ofwhich has been adjusted by multiplying by a coefficient. Because thepeak height in the first full time period (930) did not exceed the restthreshold, the peak height for that time period is not incorporated inany of the averages. In this data segment (910), the first time periodafter second 403 (1030) is the only time period in which the peak heightdid not exceed its peak threshold. Time period 1030 is shaded toindicate that it has been eliminated from consideration both as a stepand for calculation of amplitude thresholds. Thus, there are two timeperiods from this data segment that are not be considered for theamplitude threshold.

FIG. 11 illustrates the application of an amplitude threshold to a datasegment (910) after the application of a peak threshold and a restthreshold, according to certain embodiments. Each of the remaining timeperiods not disqualified by the first two thresholds contains twohorizontal lines. The solid horizontal line represents the amplitude forthe time period in which it is shown—the difference between the highestand lowest points in the time period. The dashed horizontal linerepresents the amplitude threshold for the time period in which it isshown. The amplitude threshold is the average of the last five countedamplitudes. The time periods whose amplitudes did not exceed theamplitude threshold (1130) are shaded gray (in addition to the timeperiods that did not pass the peak threshold (1030) and rest threshold(930)). In the data from this time period (910), 11 of the 24 timeperiods passed all three thresholds. Thus, eleven steps are recorded forthe eight seconds of data.

Implementation of Use:

As explained in the Background section, a single fixed threshold may notaccurately count steps of different individuals or of a singleindividual in different circumstances. One situation in which theintensity of an individual's steps may change over time involves thehealing of post-surgical patients. Research has shown that post-surgicalactivity, ambulation in particular, may increase the rate at whichpatients heal. After surgery, patients may be weak and are often inpain. Jarring, high-acceleration steps may increase their pain,particularly at incision sites. Therefore, soon after surgery, patientsoften take steps that create only low levels of acceleration, sometimesreferred to as the “hospital shuffle.” Commercial activity trackersdesigned to promote athletic fitness have failed to detect steps of thehospital shuffle. Correspondingly, activity trackers with a lower fixedthreshold that can detect steps of the hospital shuffle haveover-counted steps of healthy individuals.

As patients heal after surgery, the intensity of their steps oftenincreases as pain decreases and their strength returns. Accordingly,implementations of the present disclosure may provide systems anddevices that detect steps of varying intensity. This enables a singledevice that dynamically changes its sensing thresholds to detect auser's steps. For instance, after surgery, a device according to thepresent disclosure may detect steps of the hospital shuffle with a setof thresholds that are low due to the surrounding acceleration readings.As the intensity of a patient's steps increases, the step thresholdsincrease according to the increased magnitude of acceleration readingscaused by the increasingly vigorous steps.

In the context of a post-surgical walking program, it is advantageous toprovide each patient with a single device rather than exchanging devicesas the intensity of their steps increases. To further complicatematters, patients may recover at different rates, so the timing at whichan exchange would be needed may vary. A single device that changesthresholds for step detection allows a program to issue a single devicethat accurately counts steps throughout the progression of the patient'srecovery.

The above description of the invention is neither exclusive norexhaustive and is intended neither to describe all possible embodiments(also referred to as “examples”) of the invention nor to limit the scopeof the invention. Embodiments of the invention may include elements inaddition to those in the described embodiments and, in some cases, maycontain only a subset of the elements described in a particularembodiment. Embodiments may contain any combination of elements in thedescribed embodiments in addition to elements not expressly described.As used herein, the articles “a” and “an” may include one or more thanone of the noun modified by either without respect to use of otherphrases such as “one or more” or “at least one.” The word “or” is usedinclusively unless otherwise indicated. Terms such as “first,” “second,”“third” and so forth are used as labels to distinguish elements and donot indicate sequential order unless otherwise indicated. In addition tothe embodiments described above, embodiments of the invention includeany that would fall within the scope of the claims, below.

What is claimed is:
 1. A system for counting steps, the systemcomprising: a processor in communication with a first accelerometer anda second accelerometer, wherein (a) the first accelerometer generatesfirst accelerometer outputs at least every one third of a second frombeing coupled to an individual, the first accelerometer measures linearacceleration at least along an anterior-posterior axis of theindividual, and (b) the second accelerometer generates secondaccelerometer outputs at least every one third of a second from beingcoupled to the individual; and a non-transitory computer-readable mediumin communication with the processor, the non-transitorycomputer-readable medium storing instructions for execution by theprocessor to: determine a plurality of time periods, each of theplurality of time periods being a time period; determine readings usingthe first accelerometer outputs and the second accelerometer outputs;identify counted peak heights, each of the counted peak heightscomprising a maximum reading during a time period in which the maximumreading is greater than a rest threshold, the rest threshold determinedby: receiving readings from the first accelerometer or the secondaccelerometer during a period of rest, identifying a rest maximum, therest maximum comprising the greatest reading generated by the firstaccelerometer or the second accelerometer during the period of rest, anddetermining the rest threshold using the rest maximum; determine a peakthreshold for each of the plurality of time periods having a countedpeak height, the peak threshold comprising a first weighted average oftwo or more counted peak heights; identify counted amplitudes, each ofthe counted amplitudes comprising a difference between the maximumreading and a minimum reading during a time period in which the countedpeak height exceeds the peak threshold; determine an amplitude thresholdfor each of the plurality of time periods having a counted amplitude,the amplitude threshold comprising a second weighted average of two ormore counted amplitudes; and increment a counter if the countedamplitude exceeds the amplitude threshold associated with the timeperiod in which the counted amplitude occurred, a number stored by thecounter representing a number of steps taken by the individual.
 2. Thesystem of claim 1, wherein the processor applies a data-smoothingfunction to the first accelerometer outputs and the second accelerometeroutputs to determine the readings.
 3. The system of claim 2, wherein thedata-smoothing function comprises one or more of a low-pass filter, amoving average, r a curve-fitting function.
 4. The system of claim 1,wherein each of the plurality of time periods is of equal duration. 5.The system of claim 1, wherein each of the plurality of time periods ismeasured by a device selected from the group consisting of theprocessor, the first accelerometer, the second accelerometer, and anelectric clock.
 6. The system of claim 1, wherein determining the restthreshold comprises one or more of multiplying the rest maximum by acoefficient greater than or equal to one, or adding a fixed value to therest maximum.
 7. The system of claim 1, wherein the two or more countedpeak heights are the two or more counted peak heights immediatelypreceding each time period having a counted peak height.
 8. The systemof claim 1, wherein each of the counted amplitudes comprises adifference between the maximum reading and the minimum reading during atime period in which a function of the counted peak height exceeds thepeak threshold, the function comprising one or more of: multiplying thecounted peak height by a coefficient or adding a fixed value to thecounted peak height.
 9. The system of claim 1, wherein the two or morecounted amplitudes are the two or more counted amplitudes immediatelypreceding each time period having a counted amplitude.
 10. The system ofclaim 1, wherein the system further comprises a display screen incommunication with the processor, the display screen displayinginformation including the number stored by the counter.
 11. The systemof claim 1, wherein the second accelerometer measures linearacceleration along three axes and the first and second accelerometeroutputs from the three axes are combined.
 12. A method of counting stepsfrom outputs of a first accelerometer and outputs of a secondaccelerometer, the first accelerometer measuring linear acceleration atleast along an anterior-posterior axis of an individual, the firstaccelerometer and the second accelerometer coupled to an individual andin communication with a processor, the processor in communication with anon-transitory computer readable medium, the non-transitory computerreadable medium storing instructions for execution by the processor toexecute the steps of the method, the method comprising: determining aplurality of time periods, each of the plurality of time periods being atime period; determining readings using the outputs of the firstaccelerometer and the outputs of the second accelerometer the outputs ofthe first accelerometer and the outputs of the second accelerometerbeing generated at least every one third of a second; identifyingcounted peak heights, each of the counted peak heights comprising amaximum reading during a time period in which the maximum reading isgreater than a rest threshold, the rest threshold determined by:receiving readings from the first accelerometer and the secondaccelerometer during a period of rest, identifying a rest maximum, therest maximum comprising a greatest reading generated by the firstaccelerometer or the second accelerometer during the period of rest, anddetermining the rest threshold using the rest maximum; determining apeak threshold for each of the plurality of time periods having acounted peak height, the peak threshold comprising a first weightedaverage of two or more counted peak heights; identifying countedamplitudes, each of the counted amplitudes comprising a differencebetween the maximum reading and a minimum reading during a time periodin which the counted peak height exceeds the peak threshold; determiningan amplitude threshold for each of the plurality of time periods havinga counted amplitude, the amplitude threshold comprising a secondweighted average of two or more counted amplitudes; and incrementing acounter if the counted amplitude exceeds the amplitude thresholdassociated with the time period in which the counted amplitude occurred,a number stored by the counter representing a number of steps taken bythe individual.
 13. The method of claim 12, wherein the processorapplies a data-smoothing function to the outputs of the firstaccelerometer and the outputs of the second accelerometer to determinethe readings.
 14. The method of claim 13, wherein the data-smoothingfunction comprises one or more of a low-pass filter, a moving average,or a curve-fitting function.
 15. The method of claim 12, wherein each ofthe plurality of time periods aye is of equal duration.
 16. The methodof claim 12, wherein determining the rest threshold comprises one ormore of: multiplying the rest maximum by a coefficient greater than orequal to one; or adding a fixed value to the rest maximum.
 17. Themethod of claim 12, wherein the two or more counted peak heightsimmediately precede each time period having a counted peak height. 18.The method of claim 12, wherein each of the counted amplitudes comprisesa difference between the maximum reading and the minimum reading duringa time period in which a function of the counted peak height exceeds thepeak threshold, the function comprising one or more of: multiplying thecounted peak height by a coefficient or adding a fixed value to thecounted peak height.
 19. The method of claim 12, wherein the two or morecounted amplitudes immediately precede each time period having a countedamplitude.